Camilla said her husband, who started cancer treatment earlier this week, was "very touched" by all the messages of support he has been receiving from the public.
Camilla spoke as she was introduced to members of the Wiltshire Air Ambulance service at a musical evening at Salisbury Cathedral to celebrate the work of local charities.
Speaking to Ben Abbott, 40, critical care paramedic, who asked how the King was, she said, “He is doing extremely well under the circumstances.
“He is very touched by all of the letters and messages the public have been sending from everywhere. That’s very cheering.”
Charles has postponed all public-facing duties, but is continuing with behind-the-scenes work on his red boxes of state papers and has taken a private call from Rishi Sunak.
Charles has mainly spent his time at Sandringham following his diagnosis which was announced by Buckingham Palace on Monday as he recived his first treatment in London.
He hosted a whirlwind visit from his son the Duke of Sussex at Clarence House on Tuesday before returning to the Norfolk estate.
The Queen left Charles there to fulfil her royal engagement in Salisbury.
With the king postponing public duties to undergo outpatient treatment and Kate not expected to return to engagements until after Easter as she recovers from abdominal surgery, the onus is on the remaining royals, especially William and Camilla, to provide the public face of the monarchy.The royal family have received many messages of support from the public during royal events over the past few days.The Prince of Wales spoke about the King’s cancer diagnosis for the first time and thanked the public for their “kind messages of support” as he returned to royal duties on Wednesday.
William’s words of praise, delivered during a fundraising gala dinner, also recognised those who wished the Princess of Wales well, as she recovers from her surgery.
He told the guests “It means a great deal to us all.”
William had taken time off to support his family after Kate’s operation on January 16, and returned to public duties once her care and recovery had settled.
William gave his pre-dinner speech at the event, raising funds for London’s Air Ambulance Charity’s new fleet of helicopters, where guests included Hollywood star Tom Cruise and Arsenal football club manager Mikel Arteta.
The prince said: “I’d like to take this opportunity to say thank you, also, for the kind messages of support for Catherine and for my father, especially in recent days. It means a great deal to us all.
“It’s fair to say the past few weeks have had a rather medical focus, so I thought I’d come to an air ambulance function to get away from it all.”
William may step in to represent his father at a forthcoming event, but no engagements are scheduled at the moment and the royal palaces will make arrangements if the need arises.